There has been a long-felt need for lightweight fiber-resin composite materials which display greater strength than currently known composites. Fiber resin composites include fibrous articles, sheets and strands (tows or yarns) which incorporate a polymer matrix embedding fiber bundles of the article, sheet or strand. One major application of these composites are materials for military airplanes.
There are two major types of fibers used in composites—chopped glass fibers and continuous fibers. Chopped glass fibers are used to make composites of relatively lower strength. These composites contain from 20% to 40% of fiber by volume, usually as a mat, as described in U.S. Pat. No. 3,713,962. Stiffer and/or stronger composites use continuous fibers in yarn form and contain more than 50% fiber by volume. Examples of stiffer fibers include graphite, polyaramid or special glass fibers. As the volume of fiber in the composite increases, however, obtaining a uniform matrix between the fibers tends to be more difficult.
Composites are often prepared via a “prepreg,” i.e. a composite precursor, in which the fibrous articles or strands are impregnated with a polymer matrix precursor, as exemplified by U.S. Pat. No. 3,784,433). Prepregs are typically placed in a mold with the fibers positioned in a desired sequence and orientation and subsequently heated under pressure to fuse or polymerize the precursor components to form the polymer matrix of the final composite. The prepreg allows control of the resin content and fiber orientation. Prepregs can be provided as collimated tapes or fabrics.
One type of polymer matrix comprises thermoset plastics or resins. Thermoset resins typically are prepared from precursor mixtures comprising an oligomer and a crosslinking reagent. To make a composite from thermosets, a fiber strand is preimpregnated with a low viscosity mixture of thermoset precursors. The low viscosity can be provided by solvent, heat or emulsification. The prepreg is then “B-staged” to increase the viscosity, in which “B-staging involves heating the prepreg to a mild temperature to initiate the cure reaction of the precursor mixture. The prepreg is cooled to stop the reaction when the mixture is sufficiently thick. A final cure step is applied to consolidate the fibers and fill gaps, and the curing usually involves the application of pressure, heat or energy. The precursor mixture reacts to form a hard, three-dimensional, cross-linked polymer matrix. Incorporating thermoset resins in composites is a relatively facile process because the starting components of thermosets are either liquid resins or solutions of the precursors. These liquids have low viscosities ranging from 100 to 5,000 centipoise which allow rapid wetting of the fibers. Yarns of glass, graphite or polyaramid are easily penetrated by the low viscosity resin to the core of the yarn, thus providing each fiber with a complete coating of polymer.
Thermoset composites suffer from several disadvantages, however. Complex and long cure cycles may be necessary to produce polymer matrices with a low number of voids, as exemplified by the process of U.S. Pat. No. 5,954,898. Other processes use low molding pressures to avoid damage to the fibers. These low pressures, however, make it difficult to suppress the formation of bubbles within the composite which can result in voids or defects in the matrix coating. Thus, most processing problems with thermoset composites are concerned with removing entrained air or volatiles so that a void-free matrix is produced. Thermoset composites made by the prepreg method require lengthy cure times with alternating pressures to control the flow of the resin as it thickens to prevent bubbles in the matrix. Some high volume processes, such as resin infusion avoid the prepreg step but still require special equipment and materials along with constant monitoring of the process (e.g. U.S. Pat. Nos. 4,132,755, and 5,721,034). Thermoset polymers are not easy to process, regardless of whether the resin is applied to the yarns before molding or is infused into a preform of fibers. Although thermoset polymers have enjoyed success as in lower performance composites, the difficulties in processing these resins has restricted their application, particularly for high performance use.
To overcome some of the disadvantages of thermosets, the use of thermoplastic resins as a polymer matrix in composites has been attempted. In addition, composites formed from thermoplastics should generally be stronger than those formed from thermosets. Thermoplastic resins are long chain polymers of high molecular weight. These polymers are highly viscous when melted and are often non-Newtonian in their flow behavior. Thus, whereas thermosets have viscosities in the range of 100 to 5,000 centipoise, thermoplastics have melt viscosities ranging from 5,000 to 20,000,000 centipoise, and more typically from 100,000 to 20,000 centipoise.
Despite a viscosity difference of three orders of magnitude between thermosets and thermoplastics, the same general processes have been applied to both types of matrices for preparing composite materials. In particular, the prior art has relied on a process of (1) melting the thermoplastic; and (2) forcing the melted thermoplastic through a fibrous material, i.e. the use of melt flow. Causing melt flow of highly viscous thermoplastics requires the application of high temperatures and/or pressures which can be inherently destructive to the fibers, resulting in fiber damage or distortion. In addition, it has yet to be shown that melt flow in itself is capable of forming a void-free matrix, particularly when the matrix is used to embed individual fibers.
More specifically, most of the processes used to form thermoplastic prepregs involve coating the outside of the fiber bundles with a thermoplastic polymer powder rather than coating individual fibers. The polymer powder is then melted to force the polymer around, into and onto the individual fibers.
This process can suffer the disadvantages of melt flow as described above. The long pathway from the outside of the strand to the gaps between the fibers renders filling these gaps highly unlikely. A few processes apply melt directly to the fibers. A tape can be made by coating a dry tape of collimated fibers with the polymer and applying a heated process that forces the polymer into and around the fibers (e.g., see U.S. Pat. Nos. 4,549,920 and 4,559,262). Again, melt flow is required to form the matrix, and a void-free matrix is not assured.
Other processes for incorporating thermoplastics in composites involve preparing a thermoplastic slurry and melting and forcing the slurry onto the yarn (U.S. Pat. No. 5,019,427). A few thermoplastics can be dissolved and introduced into the fiber bundle as a solution. Removal of solvent presents extra processing problems, however. Alternatively, U.S. Pat. No. 5,725,710 describes pretreating the fibers with a dilute dispersion to ease the passage of the melt polymer in a subsequent pultrusion step to make a tape prepreg. Another process involves commingling, in which structural fibers such as graphite or glass are mixed with a thermoplastic fiber and the subsequent hybrid yarn is woven into a fabric to be molded later (e.g., see U.S. Pat. Nos. 5,355,567, 5,227,236 and 5,464,684). Separate yarns of thermoplastic and reinforcement containing many thousands of filaments, however, cannot be mixed mechanically in a one-by-one arrangement of each fiber. The fibers, at best, are dispersed as smaller bundles. The laminates produced by this process typically contain areas that are resin rich and other areas that are mainly fiber and hence void-containing. Commingled thermoplastics are also restricted to only those polymers which form fibers.
Some coating processes, such as the process described in U.S. Pat. No. 4,626,306, involve forcing large particles into a strand. Other processes involve opening the strand to accept large particle, as described in U.S. Pat. No. 5,275,883. Opening the strand and closing the strand after the addition of polymer particles adds extra steps to the coating process. In addition, mere coating of strands inherently results in an insufficient amount of polymer to form a void-free matrix.
In general thermoplastic composites have had limited success to date, due to a variety of factors including high temperatures, high pressures, and prolonged molding times needed to produce good quality laminates. Most of the efforts have been focused on combining high performance polymers to structural fibers which has only exacerbated the process problems.